This invention relates to an apparatus and method for staged power load transfer, with particular application in agricultural operations. As described below, the apparatus and method also has application in many other fields, as well. Many modern agricultural systems, including ventilation systems, controlled environment housing, computerized controls and many other applications require a nearly constant supply of electricity. More automation has resulted in changes in how emergency back-up power is supplied. P.T.O. generators have been in use for many years, and remain in use on many farms today. However, many farms have changed to engine-driven back-up generators with automatic transfer switches. These systems offer many advantages to the farmer beyond providing an emergency source of power, including quicker response time to power outages, automatic operation, remote operation, increased flexibility and the ability to participate in utility load management options.
These new systems require a different approach to load management during and after load transfer, particularly since most of the these systems are electronically controlled, often by microprocessors, and are thus very sensitive to anomalies such as power surges, dips, transients and voltage fluctuations. The more sophisticated the control, the more sensitive it is to these problems.
Electrical utilities strive to provide uninterrupted electricity within a relatively narrow range of standard characteristics. However, constant adherence to these narrow standards is impossible in situations where hundreds of customers are served over dozens or hundreds of miles of power lines from the same circuits. An electrical failure or disturbance at one customer""s facility can affect all of the other customers served from the same circuits. Large motor stops and starts, capacitor switching and other circuit operations create line disturbances that can effect the operation of electronic devices. Utilities require large motors to be equipped with devices which limit current inrush (starting kva) so that distribution line voltage will not dip below a certain minimum value. These same problems can occur at a single site, for example, a poultry farm, when a utility interruption causes a switch to back-up power. Even though most motors, such as fan motors, are relatively small, collectively they can add up to a very large drain on current supplied by a generator. Therefore, one embodiment of the present invention xe2x80x9cstagesxe2x80x9d the start-up of motors to limit current inrush and prevent voltage dips and surges. In addition, one embodiment of the present invention enhances smooth starting by using a relay which energizes at the exact time of xe2x80x9czero crossing,xe2x80x9d, i.e, when the instantaneous supply voltage is at zero. Finally, the present invention utilizes a fast response load isolation that protects motor loads and electronics during power switching transitions.
Therefore, it is an object of the invention to provide a power load transfer circuit apparatus and method.
It is another object of the invention to provide a power load transfer circuit which includes staged load addition to a power supply.
It is another object of the invention to provide a power load transfer circuit which uses a relay which energizes at the exact time of zero crossing when the instantaneous supply voltage is at zero.
It is another object of the invention to provide a power load transfer circuit which uses a relay which turns off at the exact time of zero crossing when the instantaneous supply voltage is at zero.
It is another object of the invention to provide a power load transfer circuit which utilizes fast response load isolation that protects motor loads and electronics during power switching transitions.
These and other objects of the present invention are achieved in the preferred embodiments disclosed below by providing an apparatus for isolating electrical loads during a power supply transfer interruption event, comprising an electrical circuit interposed between an electrical power source and a load which comprises a voltage detector for continuously detecting voltage supplied to the load from the power source, comparing the detected voltage against a predetermined desired voltage and outputting a signal indicative of an out-of-limits voltage condition from the power source to the load, an interrupt switch for receiving the output from the voltage detector and interrupting the supply of power to the load during an interval of no more than 1 cycle of loss of the normal power supply after detecting the out-of-limits voltage condition, and a reset switch for restoring the power supply to the load.
According to one preferred embodiment of the invention, the power load transfer circuit includes a zero-crossing circuit for permitting the switch to interrupt the power supply only at the zero-crossing point of the power being supplied to the load.
According to another preferred embodiment of the invention, the reset switch includes a delay circuit for delaying restoration of power to the load after an interruption of power to the load.
According to yet another preferred embodiment of the invention, the reset switch comprises a variable resistor in the form of a potentiometer for permitting the delay to be varied in accordance with a predetermined optimum time for power restoration after interruption.
According to yet another preferred embodiment of the invention, an apparatus is provided for isolating electrical loads during a power supply transfer interruption event, including an electrical circuit interposed between an electrical power source and a plurality of loads comprising a plurality of voltage detectors for continuously detecting voltage supplied to the plurality of loads from the power source, comparing the detected voltage against a predetermined desired voltage and outputting a signal indicative of an out-of-limits voltage condition from the power source to any one of the plurality of loads, an interrupt switch for receiving an output from the voltage detectors and interrupting the supply of power to the loads in no more than 1 cycle of the power supply from the point of the out-of-limit voltage detection, and a reset switch for restoring the power supply to the loads.
According to yet another preferred embodiment of the invention, the power load transfer circuit includes a plurality of switches connected to respective ones of the loads, each of the switches including a delay circuit for delaying restoration of power to the loads after an interruption of power to the loads.
According to yet another preferred embodiment of the invention, at least some of the delay circuits includes a variable timer for varying the time of activation of the reset switches relative to each other as desired to permit the power to be restored to the loads in a predetermined sequence.
According to yet another preferred embodiment of the invention, each of the delay circuits include a variable timer.
A method for isolating electrical loads during a power supply transfer interruption event according to an embodiment of the invention comprises the steps of continuously detecting a voltage supplied to the load from a power source, comparing the detected voltage against a predetermined desired voltage, outputting a signal indicative of an out-of-limits voltage condition from the power source to the load, interrupting the supply of power to the load during an interval of no more than 1 cycle of the power supply from the point of the out-of-limit voltage detection, and restoring the power supply to the load upon termination of the out-of-limits voltage condition.
According to yet another preferred embodiment of the invention, the method includes the step of permitting the interruption of the power supply only at the zero-crossing point of the power being supplied to the load.
According to yet another preferred embodiment of the invention, the method includes the step of restoring the power supply to the load includes the step of delaying restoration of power to the load after an interruption of power to the load.
According to yet another preferred embodiment of the invention, the step of restoring the power supply to the load comprises the step of varying the delay in restoration of the power supply in accordance with a predetermined optimum time interval for power restoration after interruption.
A method for isolating a plurality of loads during a power supply transfer interruption event according to the invention comprises the steps of continuously detecting voltage supplied to the plurality of loads from the power source, comparing the detected voltage against a predetermined desired voltage, and outputting a signal indicative of an out-of-limits voltage condition from the power source to any one of the plurality of loads, interrupting the supply of power to the loads in no more than 1 cycle of the power supply from the detection of the out-of-limits condition, restoring the power supply to the loads upon termination of the out-of-limits voltage condition.
According to yet another preferred embodiment of the invention, the method includes the step of delaying restoration of power to the loads after an interruption of power to the loads.
According to yet another preferred embodiment of the invention, the method includes the step of varying the time of restoration of the power to the loads in predetermined intervals.